1. Field of the Invention
The yaw damping system and method are in the field of aircraft control systems. More specifically, the yaw damping system and method are in the field of damping systems for controlling the flight modes of aircraft, namely the dutch roll mode.
2. Description of the Related Art
When an aircraft is disturbed from steady flight it exhibits varying behavior depending on the type of disturbance and the aerodynamic characteristics of the aircraft. The types of behavior exhibited by aircraft are categorized as various aerodynamic modes, including the spiral mode and the dutch roll mode, among others. For example, the spiral mode, if not stabilized, may lead an aircraft to enter a spiral dive leading to a crash.
Aircraft may be designed to stabilize the various modes, through design elements such as the provision of dihedral wings to stabilize the spiral mode. Alternatively, the aircraft control systems may be designed to dampen or counteract instability in the modes. The system and method described herein are designed to dampen instability in the dutch roll mode.
The dutch roll mode is a mode that includes a roll oscillation with an out-of-phase yaw oscillation. The mode is excited by the use of rudder or ailerons creating a sideslip, whereby the movement of the relative wind across the aircraft has a component that is perpendicular to the longitudinal axis of the aircraft. In other words, the nose of the aircraft is not pointing directly into the relative wind.
If the aircraft has both positive lateral stability and directional stability, the sideslip produces a roll moment and a yaw moment to reduce the sideslip. These moments are the result of aerodynamic forces acting on the aircraft due to the sideslip that tend to induce both a roll moment and a yaw moment to correct the sideslip. For example, the roll moment may be induced by differences in wind speed over the wings of the aircraft, which causes the aircraft to bank in a manner that tends to remove the sideslip. The aerodynamic forces also produce a yaw moment that tends to reduce the sideslip by turning the nose of the aircraft into the relative wind. This yaw moment may be induced by pressure on the vertical stabilizer surfaces of the aircraft.
In many aircraft designs, the yaw moment is weaker than the roll moment created by a sideslip, and lags in time behind the roll moment created by the sideslip. The roll moment returns the aircraft to level non-sideslip flight before the yaw moment, and thus the yaw moment continues to turn the aircraft past level non-sideslip flight. This time lag causes the aircraft to overshoot the non-sideslip orientation and enter a sideslip in the opposite direction. The process then reverses itself and if the dutch roll mode is unstable in the given aircraft, the oscillations will tend to increase in amplitude over time.
In order to dampen an unstable dutch roll mode, aircraft may utilize a classical yaw damper to counteract the lagging yaw moment created by the sideslip. In a classical yaw damper, the yaw rate of the aircraft, adjusted by a proportional gain, is fed back to the rudder. This counteracts the yaw rate component of the aerodynamic reaction to the sideslip condition and dampens the dutch roll mode. Thus the yaw moment created by the sideslip is counteracted by control surface input that is proportional to and in opposition to the yaw moment.
The control surface input necessary to counteract the yaw moment may be generated and applied without pilot intervention by the aircraft control systems, which may continuously generate and apply the necessary input throughout operation of the aircraft.
While the classical yaw damper effectively dampens the dutch roll mode, it is susceptible to failure from a variety of faults, and many aircraft are required to provide backup mechanisms for damping the dutch roll mode. For example, if an aircraft's yaw control surfaces jam or are otherwise inoperable, the classical yaw damper will be inoperative as a result. Therefore, it is desirable to provide an alternative damper for the dutch roll mode that is not dependent on the yaw control surfaces of the aircraft, and that can independently dampen the dutch roll mode if the classical yaw damper is inoperative.
It is also desirable that any alternative damper for the dutch roll mode must be compatible with the classical yaw damper such that the two dampers may operate simultaneously without reducing the stability of the aircraft or the efficacy of the other damper mechanism. This is necessary because both damping mechanisms must be able to operate continuously throughout the operation of the aircraft. Thus in embodiments it is desirable to utilize control surfaces other than the yaw control surfaces to dampen the dutch roll mode.
The alternative yaw damper described herein provides effective independent damping of the dutch roll mode and it works in cooperation with classical yaw dampers without reducing their efficacy.